1. Field of the Invention
The present invention relates to copolymers of acrylamide and dimethylaminopropylacrylamide as flocculating agents, and a process for dewatering sludges which are generated during the mechanical or biological purification of municipal and industrial waste water by use of such agents.
2. Description of Related Art
The purification of waste water necessarily results in the generation of sludge. Depending on the purification process that is used, this sludge generally consists for the most part of water, with organic material dispersed therethrough.
A typical settled sludge from a municipal sewage treatment plant contains 95% water, of which approximately 70% is present in the form of cavity water, approximately 22% is present as capillary water, and approximately 8% is present as internal water. As a consequence of the high proportion of organic components, such a sludge is stabilized by negative charges on the surfaces of the particles and a reduction in the proportion of water is only possible by the use of powerful physical or chemical forces.
During the machine dewatering of turbid water, a flocculating agent is often used to accelerate the removal of the water. In the case of solid/liquid systems, such as are used in the purification of sludge, these flocculating agents are usually polymers of cationized acrylic acid derivatives or methacrylic acid esters or copolymers of these esters with acrylamide.
The use of these flocculating agents leads to a destabilization of the solid particles by neutralization of the surface charges and an agglomeration of the smaller particles, which results in formation of larger floccules. The cavity water that is liberated when this occurs as well as some of the capillary water can be removed by the use of conventional dewatering machines such as travelling screen presses, centrifuges, or chamber filter presses. However, depending on the type of sludge and the selection of the dewatering system, the sludge cakes that are formed still contain 60-85% water.
Polymers based on cationized acrylic acid or methacrylic acid esters, used for flocculating sewage sludge, display a number of disadvantages. For use as flocculating agents, such polymers must be the form of diluted 0.1 to 0.3% aqueous solutions. Because of the ester groups that are susceptible to hydrolysis, these solutions can only be stored for a limited time. In aqueous solvents with pH values from 7.0 to 7.5, acrylic derivatives are stable for only a few hours, and in the case of methacrylic derivatives, approximately 24 hours.
A further problem is encountered during the flocculation and dewatering of alkaline sewage sludge, which occurs during the stabilization of such sludge with lime or, for example, when the sludge is heated to approximately 80 to 90.degree. C. Here, too, the effective use of the flocculating agents discussed above becomes impossible because of the hydrolysis of the esters.
Conventional dewatering systems such as centrifuges, travelling sieve presses and chamber filter presses each impose other demands on the conventional flocculating agents. Because of their heavy power requirements, which can amount to some 30,000 W/m.sup.3 of sludge, centrifuges require large shear resistant floccules which are as a rule formed by extremely high molecular weight flocculating agents. By contrast, the special demands encountered during dewatering using travelling band presses are satisfied by floccules that are stable under pressure and fulling conditions, preferably formed by polymers having a low degree of polymerization, and which are protected by the use of power inputs between 100 and 400 W/m.sup.3 of sludge. On the other hand, chamber filter presses require floccules that are stable under pressure and which drain well. This is achieved by the use of high molecular weight flocculating agents and relatively gentle conditioning.
The conventional flocculating agents that are based on (meth)-acrylic acid esters are not only susceptible to hydrolysis, but their effectiveness also depends on the conditions under which processing is carried out. That is to say that products with which it is possible to achieve an adequate flocculating effect by the use of gentle processing conditions with low power inputs fail under other conditions that require the inputs of larger amounts of power. For this reason, the conventional flocculating agents have to be specially adapted to the particular conditions under which the processing is carried out. Thus, a "universal" flocculating agent for the various dewatering systems would be desirable but, heretofore, it has been impossible to find such an agent amongst the conventional products.
U.S. Pat. No. 3,014,896 describes copolymers of dimethylaminopropylacrylamide (DMAPAA) and acrylamide (ACA) and their use as supplementary agents for dewatering sludge. However, with these agents, the maximum achievable viscosity for a 25% aqueous polymer solution only reaches 60 mPa.s. The DMAPAA of these copolymers is preferably neutralized with acetic acid, since mineral acids result only in low molecular weight polymers with a correspondingly low flocculating effectiveness.